[PATCH 2/3] tcp: Properly propagate tap-side RST to socket side

[David Gibson <david@gibson.dropbear.id.au>]

When the guest sends a TCP RST, or on certain error conditions, we want to
signal the abnormal termination of a TCP connection to the peer with an
RST as well.  We attempt to do that by close()ing the socket.

That doesn't work: a close() will usually send a FIN, rather than an RST.
The standard method of forcing an RST on a socket is to set the SO_LINGER
socket option with a 0 timeout, then close().

Update the tcp_rst() path to do this, so it forces a socket side RST.
Update the handling of a guest side RST to use the same path (minus
sending a tap side RST) so that we properly propagate guest RSTs to the
peer.

Link: https://bugs.passt.top/show_bug.cgi?id=191

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/tcp.c b/tcp.c
index 45dde5a0..9da37c2f 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1403,7 +1403,34 @@ static int tcp_send_flag(const struct ctx *c, struct tcp_tap_conn *conn,
 }
 
 /**
- * tcp_rst_do() - Reset a tap connection: send RST segment to tap, close socket
+ * tcp_sock_rst() - Close TCP connection forcing RST on socket side
+ * @c:		Execution context
+ * @conn:	Connection pointer
+ */
+static void tcp_sock_rst(const struct ctx *c, struct tcp_tap_conn *conn)
+{
+	const struct linger linger0 = {
+		.l_onoff = 1,
+		.l_linger = 0,
+	};
+
+	/* Force RST on socket to inform the peer
+	 *
+	 * We do this by setting SO_LINGER with 0 timeout, which means that
+	 * close() will send an RST (unless the connection is already closed in
+	 * both directions).
+	 */
+	if (setsockopt(conn->sock, SOL_SOCKET,
+		       SO_LINGER, &linger0, sizeof(linger0)) < 0) {
+		flow_dbg_perror(conn,
+				"SO_LINGER failed, may not send RST to peer");
+	}
+
+	conn_event(c, conn, CLOSED);
+}
+
+/**
+ * tcp_rst_do() - Reset a tap connection: send RST segment on both sides, close
  * @c:		Execution context
  * @conn:	Connection pointer
  */
@@ -1412,8 +1439,10 @@ void tcp_rst_do(const struct ctx *c, struct tcp_tap_conn *conn)
 	if (conn->events == CLOSED)
 		return;
 
+	/* Send RST on tap */
 	tcp_send_flag(c, conn, RST);
-	conn_event(c, conn, CLOSED);
+
+	tcp_sock_rst(c, conn);
 }
 
 /**
@@ -1884,7 +1913,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 			return -1;
 
 		if (th->rst) {
-			conn_event(c, conn, CLOSED);
+			tcp_sock_rst(c, conn);
 			return 1;
 		}
 
@@ -2248,7 +2277,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 	flow_trace(conn, "packet length %zu from tap", l4len);
 
 	if (th->rst) {
-		conn_event(c, conn, CLOSED);
+		tcp_sock_rst(c, conn);
 		return 1;
 	}
 
-- 
2.52.0